This is a renewal application which proposes to extend the finding of the first funding period to a clinical test of antiviral management and correlative research using a treatment algorithm based on individualized CMV risk factors. The model uses the high rate of CMV reactivation in recipients of hematopoietic cell transplantation (HCT) to study the innate and adaptive immune factors which govern this event in those with and without such reactivation. The study to date has demonstrated that it is the innate immune system rather than the adaptive immune system which controls CMV reactivation. Specifically, the presence in the donor genotype of activating KIR (aKIR) genes, aKIR2DS2 and aKIRDS4, are highly predictive of how well the recipient will ultimately control future CMV infection. A known failure rate in persons who otherwise have a protective donor aKIR genotype offers a model for dissection of molecular factors which affect NK control of this specific virus infection. Conversely, the protective role of other NK receptors, such as NKG2, will be sought in patients without CMV reactivation. The hypothesis of this study is that the innate responses control CMV reactivation and when they fail, it is due to alterations in genetics or in expression of NK receptor genes. There are two aims 1) continuation of the clinical immunological study of the CMV Model in HCT recipients managed using CMV risk factors to guide preemptive antiviral therapy, and 2) the laboratory assessment of natural killer (NK) cell immunophenotypes and functions, with emphasis on characterization of mRNA levels of aKIR genes and definition of promoter sequences. Assessment of the appearance and function of NK cells at various times post-HCT will be made relative to virologic events. This could lead to a better method for patient management post-HCT, and a better understanding of the role of KIR and lectin-based NK receptors, and factors associated with their silencing, will advance this important aspect of immunity.